1. Field of Invention
The present invention relates to a handling method and system for ship construction and repair especially for the removal or installation of the propeller shafts, propellers, rudder and other heavy, large objects on the underside of the hull of a large ship.
2. Description of the Prior Art
Hydrodynamic efficiency, strength and functional capability are the principal design considerations that establish size, location and surface contours of ship propellers, propulsion shafts and rudders. Little, if any, consideration is incorporated into the design to facilitate the removal or installation of these parts or to improve the shipyard handling. There is also little standardization in the size of parts in order to meet different design criteria for different types of ships. The propellers, shafts, and rudders of a supertanker meet very different design criteria than those for a high speed naval vessel or those of a small freighter.
Accordingly, handling methods, which have been generally very labor intensive, evolved to accommodate wide variations in shape and size of the major components. The most common method employs many pad-eyes welded to the underside of the hull from which chain falls are attached and extend downward to be secured to the component, i.e. the propeller, shaft or rudder. These components are then moved by continuously adjusting the chain falls and moving the components along the chain falls until the main shipyard overhead crane can secure the component and move it. The shipyard crane can only reach the component when the component is moved from under the hull.
A variety of adjustable dollies and support cribbing has also been used but these methods proved unsatisfactory in part, because of the wide variations in component sizes and the variations in hull shape. With some ships, the propeller shafts, for example, are quite high off the drydock floor. The same ship, however, may have a shaft located closer to the keel that will be somewhat nearer to the drydock floor. It is often very different to move in the close locations. Therefore, typical forklift trucks and variations have proved unsatisfactory.
Many of the past techniques rely on building scaffolding under the ship, but scaffolding interferes with free movement of materials. Moreover, scaffolding might also interfere with work platforms around the vessel.
Propeller shafts create particular problems because of their length and weight. Some exceed 40 feet (12 m), and they may have a maximum diameter of approximately 24 inches (61 cm). (All metric conversions are approximate.) The shafts may weigh up to about 60 tons (54 metric tons). Drydock floors are not always smooth and level. Many have deep, wide grooves for recessing bilge block chains. For example, one shipyard has 6.times.18 inch (15.times.45 cm) wide grooves at about 10 foot (3 m) intervals. A typical wheeled forklift has difficulty functioning with the grooves. Also, forklift, wheeled chassis tend to be built high.
One previous system uses heavy-duty electric forklift trucks having a high lift platform. Special pallets are fixed to the platform of the forklift truck. Each platform has an approximately 10 foot (3 m) square channel frame divided into two sections. Each section has a sliding bridge that spans the width of the frame, and a carriage slides inside each bridge. The top of each carriage has a telescoping mast with specialized load arms on top. One load arm handles small propellers, another holds small rudders, and another has gimbals and a saddle for handling propeller shafts. The bridges within the pallet, the carriagse within the bridges, and the telescoping uprights on the carriages are each powered hydraulically to move in two directions. This combined movement provides for four-way movement of each upright in the horizontal plane and two-way movement of the load arms on the uprights.
For long propeller shafts, two trucks with separate pallets are used, and their movement are coordinated. For installing or removing large rudders or propellers, two trucks travelling abreast must be used to pick up a saddle assembly that holds the rudder or propeller. Because of height limitations imposed on the system, it is difficult to move propeller shafts that are mounted near the drydock floor and on the same piece of equipment reach propeller shafts located very high. It is also difficult to coordinate the movement of the telescoping arms and the trucks, especially when they are required to move together. When carrying rudders or propellers between two trucks, the load on each truck is also well off the center of gravity, which tends to tilt both trucks toward each other.
For proper movement, propeller shafts should seat properly in the carriage, but it is somewhat time consuming to align the entire truck perfectly. Therefore, it would be desirable to provide some play in the carriage so that the device is self-aligning.
The wide variety of propeller sizes and their varied rake and pitch causes significant difficulties in handling propellers. Where the rudder or propeller are positioned near the back wall of the drydock, movement of handling systems is frequently restrictive.